Unequal distribution of pulmonary diffusing capacity in the anesthetized dog

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Abstract
In anesthetized dogs the alveolar-arterial O2 pressure difference (AaD) was measured at alveolar O2 pressures of 45, 75, 106, 146, and 255 mm Hg. The AaD values observed could not be explained by the conventional “shunt factor,” “diffusion-limitation factor,” or “distribution factor.” However, the experimental data could be explained on the basis of the concept of unequal distribution of pulmonary diffusing capacity, D, to perfusion, Q. A procedure for estimation of the pattern of distribution of D to Q from experimental data is described. The results were compatible with the assumption that the lung consisted of a minimum of three functional compartments characterized by different D/Q ratios. A small portion of the perfusion (1.5%) probably behaved like a true shunt (D/Q = 0); 13% of the pulmonary blood flow passed through a compartment that shared in 2% of the total diffusing capacity only, resulting in a relatively small D/Q ratio for this compartment. The presence of this compartment gave rise to the largest part of the AaD at air breathing. The calculated D/Q value for the remaining, major compartment was so high that a measurable AaD due to diffusion limitation in this compartment could occur only at alveolar O2 pressures lower than 60 mm Hg. The validity of the assumptions and the significance of the results are discussed. Submitted on May 23, 1960